The meaning of the first is precise, so I'll only comment on #2. "That she plays the piano" is a fact. You cannot see a fact, so "seen" can only have a metaphorical meaning. You can see PROOF or EVIDENCE that she plays the piano, but you cannot see a fact. The metaphorical meaning for "seen" that makes sense is something like
"seen" = "seen proof" or
"seen" = "seen evidence" or
"seen" = "realized" or
"seen" = "become aware".
Sight is not the only sense that can be used in such a question. You could also use any of the words "heard", "read", "learned", "noticed", or many others.
Both are excellent sentences. The second sentence is a question about knowledge, whereas the first is a straightforward question about experience.